Morphological Growth and Theoretical Understanding of Gold and Other Noble Metal Nanoplates

For the last decades, the chemical reduction of Au³⁺ to Au⁰ has been widely employed to produce various gold nanostructures. In comparison with the fast reduction, the slow reduction is systematically investigated in this research to provide more insights to reveal intermediary process and further disclose the underlying mechanism for growing gold nanostructures by using a series of simple ligands with aldehyde groups as weak reducing agents. The different binding energies of ligands to Auⁿ⁺ (n=3, 1 and 0) exhibit variable binding affinities in starting, intermediate, and final gold species. For example, formic acid has much stronger binding affinity to Au⁺ than Au³⁺, and thus Au⁺ intermediate is able to be stabilized/captured during slow reduction of Au³⁺. Upon the disproportionation of Au⁺ to Au⁰ and Au³⁺, formic acid has much stronger binding affinity to the newly formed Au⁰ than other ligands for the controlled formation of gold nanostructures. Meanwhile, the adsorption of ligands causes substantially decreased surface energies on different gold planes. There are much higher energies on {110} planes compared to the other two {111} and {100} planes with certain ratios in these energies, leading to morphological growth of gold nanosheets. In this paper, we experimentally demonstrate anisotropic growth of gold nanosheets by using various ligands with weak reducing and appropriate coordination capabilities, and further provide insights to understand their morphological growth mechanism behind. This synthetic strategy is successfully extended to prepare silver, palladium, and platinum nanoplates.